1. Field of the Invention
The present invention relates generally to a permanent magnet rotating electric machine and an electrically driven vehicle employing same.
2. Description of Related Art
Motors used in electrically driven vehicles, in particular, driving electric cars must ensure a sufficient running distance with a limited battery capacity, so that they are desired to be small, light-weight, and highly efficient.
For a motor to be small and light weight, it is required to be suitable for high speed rotation. In this regard, permanent magnet motors are advantageous over direct-current motors and induction motor.
Permanent magnet rotors are classified into a surface magnet rotor which has permanent magnets positioned along the outer periphery of the rotor and a so-called internal magnet rotor which has a permanent magnet holder within a core made of silicon steel or the like having a higher magnetic permeability than permanent magnets.
The surface magnet rotor is advantageous in ease of control, less influences by reactive magnetic flux of a stator winding, low noise, and so on. However, the surface magnet rotor also has several disadvantages such as requirement of reinforced magnets for high speed rotation, a narrow speed control range due to difficulties in field weakening control, a low efficiency in high speed and low load operations, and so on.
The internal magnet rotor, in turn, has advantages such as the capability of high speed rotation by field weakening control using magnetic pole pieces positioned along the outer periphery of magnets, the capability of highly efficient rotation in high speed and low load operations, utilization of reluctance torque, and so on.
Prior art internal magnet rotors are described, for example, in JP-A-5-219669, FIG. 5 of JP-A-7-39091.
Within large-size permanent magnet motors used in electric vehicles and so on, those having an internal permanent magnet rotor employ a distributed winding stator for their stator structure.
However, permanent magnet motors described in the prior art have a disadvantage that pulsating torque based on high frequency components of permanent magnets or auxiliary magnet poles is produced. Also, cogging torque is produced by influence of roughness and fineness of magnetic flux of stator salient poles and roughness and fineness of magnetic flux of permanent magnets, and smooth rotation of permanent magnet motors cannot be obtained. Further, since the distributed winding stator has elongated winding ends, this causes a limitation to reduction in size and weight of rotating electric machines employing a distributed winding stator.